The invention is in the field of video image color processing, and is particularly relevant to identification of colors in a video image signal.
In the field of video image processing, an image is scanned and rendered into a string of individual picture elements (pixels), each corresponding to an elemental portion of the image and representing the instantaneous value of optical characteristics of the portion of the image embodied in the pixel. In monochromatic television, light intensity is the attribute represented by a pixel. In digitized black and white television, a pixel is a multi-bit digital representation of light intensity. Pixels are presented serially in a standard scan format composed of lines, fields, and frames to represent an image.
A pixel in a color imaging system represents light intensity and other chromaticity characteristics which, when combined with intensity, represent the color of the portion of the image embodied in the pixel. Scanned color imaging systems correspond with monochromatic systems in that repeated concatenations of pixels represent a scanned image. Two well-known scanned color image representations are the NTSC and RGB forms.
Typical scanned color image systems are based upon one or more representations of color space. Here, color space refers to any one of a number of three-dimensional representations of all of the possible combinations of three predetermined color attributes. For example, a set of color elements can include hue, saturation, and intensity.
In U.S. Pat. No. 4,991,223 "APPARATUS AND METHOD FOR RECOGNIZING IMAGE FEATURES USING COLOR ELEMENTS", and commonly assigned with this application, a system is presented for recognition of objects in a scanned color image based upon classification of a color video signal by comparison with a set of defined colors. This patent is incorporated herein by reference.
The method and apparatus of the incorporated patent is based upon the concept of color as a location in three-dimensional space defined by color axes, and the optional concept of transforming the axes in color space to yield one axis corresponding to brightness (intensity) and two other axes representing non-intensity color elements. The system of the incorporated application can be understood with reference to FIG. 1 which shows a cube 10 representing color space, the color space being defined by three mutually orthogonal axes, 12, 14, and 16. Each orthogonal axis corresponds to a color element. The color elements can be, for example, the R-Y, B-Y, and Y channels of a color video system. The cube 10 represents all colors which the color processing system of this invention can process. In the operation of the system in the incorporated patent application, a set of colors to be identified in a video image is defined. Each defined color is established by three separate thresholds for the three channels of video color. A video color is accepted by the system of the incorporated patent as a defined color if it is within all three thresholds as determined by the system. In this regard, three thresholds in FIG. 1 are indicated by th.1, th.2, and th.3. Each of the thresholds represents a range of color element magnitudes on one of the respective axes 12, 14, or 16. The three thresholds define mutually perpendicular slabs in color space whose intersection is mapped by the rectangular space 20. The rectangular space essentially establishes the defined color which is achieved by combining the three color elements in the magnitude ranges represented by the three thresholds of FIG. 1.
The system of the incorporated patent operates effectively to discriminate between colors contained in non-intersecting rectangular solids of color space. However, there is a need to discriminate colors which might lie within a rectangular region formed by the intersection of two rectangular solids. In particular, two partially shadowed objects whose hue and brightness are the same and which differ only in saturation appear as two ovoids in color space which lie one on top of the other. This is illustrated in FIG. 2 by the ovoid solids 22 and 24. Inspection of FIG. 2 will reveal that the rectangular approach based upon the thresholds of FIG. 1 cannot establish a set of three thresholds for definition of the colors in the ovoid 22 which will always distinguish colors in a solid defined by three other thresholds which contains the colors of the ovoid 24.